Congenital abnormalities of the kidney and urinary tract (CAKUT) are the leading cause of end stage renal disease among children under 5 years, with an occurrence of approximately 1 in 500 newborns. Among the several genetic mutations that have been linked to CAKUT are mutations in Notch signaling pathway genes, Jagged-1 and Notch2. Mutations in Notch pathway genes, predicted to reduce the level of Notch signaling activity, are associated with Alagille Syndrome (ALGS), one component of which is the formation of small multi-cystic, dysplastic kidneys. Recently, the Hajdu-Cheney syndrome, also involving renal cysts, is associated with mutations in Notch2. Although the Notch pathway mutations are correlated with human pediatric syndromes including smaller kidneys and renal cysts, the underlying cellular and molecular mechanisms by which Notch signaling regulates nephron formation are only beginning to be understood. The objective of this project is to improve our understanding of the cellular and molecular mechanisms regulating renal proximal tubule morphogenesis and hence to contribute to better treatment options and better management of patients suffering from CAKUT. We propose to utilize the Notch signaling pathway as a molecular handle to identify the cellular and molecular mechanisms regulating renal proximal tubule morphogenesis. We have recently identified that reducing the level of Notch signaling during kidney development in mice results in small multi-cystic, dysplastic kidneys, proving that Notch signaling plays a critical role in renal tubule morphogenesis. In aim 1 we will use the mice with Notch signaling deficient kidneys, and three renal cell lines with Notch signaling inactivation to identify the cellular processes and genes by which Notch signaling regulates renal epithelial cell division during tubule morphogenesis. Additionally, in aim2 we will identify proteins and pathways that Notch signaling interacts with in regulating renal proximal tubule morphogenesis. This will involve testing the hypothesis that mutations in the Notch pathway genes that partially inactivate Notch signaling will modify the kidney disease in Pkd2 +/- mice. It will also involve an unbiased screen for proteins that interact with the intracellular domain of Notch2. These studies will improve our understanding of the cellular and molecular mechanisms regulating renal proximal tubule morphogenesis and in general the mechanisms by which epithelia are retained as a monolayer while they divide.